The invention relates to a wafer chuck having a substrate and having, applied to the substrate, an electrically conductive coating for fixing a wafer by electrostatic attraction and preferably having a reflective coating applied to the substrate.
In order to hold plate-like objects that are in the form of wafers, use is made of wafer chucks which position, or fix, the wafers in a suitable manner. In EUV lithography, the wafer and the wafer chuck are typically in a vacuum environment of an EUV lithography system. The wafer is typically attached to a likewise plate-shaped holder which is referred to as a wafer table. The wafer, together with the wafer table, is fixed to the upper side of the wafer chuck by electrostatic attraction. The upper side of the wafer chuck is for that purpose provided with an electrically conductive coating which acts as an electrode and which may be composed, for example, of a layer of chromium. An electrically conductive coating is here understood to mean a coating having at least one conductive layer which is electrically contactable in order to fix the wafer by electrostatic attraction. Optionally other layers which are non-conductive or only weakly conductive may be applied above or below that layer. It will be appreciated, however, that it is also possible for the entire coating to be made up of conductive layer materials.
In order further to enhance the properties of the coating with regard to their scratch resistance, it was proposed to use coating materials that had a greater coefficient of friction and a greater material hardness than chromium. Those materials are typically applied by sputter coating or by an ion-assisted process which as a rule leads to layers that have very high mechanical compressive stresses of up to a few GPa. Under the influence of those high mechanical layer stresses, the wafer chuck may become deformed which, in an extreme case, may lead to the requirements in respect of the evenness of the wafer substrate no longer being fulfilled.
Although an attempt can be made to prevent the occurrence of excessive deformation by applying the coating with as small as possible a layer thickness, the problem arises that the layer applied should not be less than a minimum of approximately 100 nm thick in order not to exceed the maximum surface resistance allowed in the case of the present applications of typically from 100 Ohm to 200 Ohm.
In addition to the electrically conductive coating, it is possible to apply, for example, to the side faces of the substrate, a reflective coating which can be used for the exact positioning of the wafer chuck, for example, with the aid of a laser beam. Even with the reflective coating, excessive layer stresses may possibly lead to an undesired deformation of the wafer chuck.
In addition, layer stresses change after introduction into a vacuum as a result of a reduction in the water content of the layers. Changes in the layer stress of the order of magnitude of a few MPa can still be observed for several days after introduction into a vacuum. Such a long-term change in the layer stress in vacuum necessitates frequent re-calibration of the wafer-chuck positioning.
In order to reduce the stresses on a titanium nitride layer or a titanium layer of a wafer, it is known from U.S. Pat. No. 5,936,307 to roughen a substrate composed of a dielectric material to which that/those layer(s) are applied.
In order to produce a coating having high wear resistance, it is known from JP 61091354 to apply a first thin layer of material to a substrate by ion-plating. A second layer of the same material is applied to the first layer by vapour deposition, the second layer having a tensile stress. A third layer is subsequently applied to the second layer by ion-plating in a reactive gas plasma. The third layer may be composed of titanium nitride, boron nitride, silicon carbide etc. and has a compressive stress.
US 2008/0153010 A1 describes the deposition of a reflective multi-layer coating on a substrate by sputtering. The multi-layer coating applied by sputtering has a layer stress which results in deformation of the substrate. In order to compensate for that deformation, the multi-layer coating is applied to a substrate which is deformed in the opposite direction so that, after the application of the multi-layer coating, the desired flat shape of the substrate with the coating results. In order to achieve deformation of the substrate, it is proposed, inter alia, to support the substrate on a wafer chuck having a curved surface.
U.S. Pat. No. 7,220,319 B2 discloses a wafer chuck having a substrate composed of a conductive material to which an electrode is fixed by screws. The electrode is delimited at the top by a first layer, the thermal expansion coefficient of which lies between that of a dielectric plate on which the wafer is supported, and the expansion coefficient of the electrode. Arranged between the electrode and the substrate is a second layer, the thermal expansion coefficient of which likewise lies between that of the dielectric layer and that of the substrate. The aim of this choice of thermal expansion coefficient is supposed to be that the layer stress of the electrode acts in an opposite manner to the layer stresses of the two layers enclosing the electrode.
JP 2001-223 261 A describes an electrostatic wafer chuck in the case of which three layers having different thermal expansion coefficients are applied between an electrically conductive substrate and an insulating covering layer in order to obtain gradual adaptation between the expansion coefficient of the substrate and the expansion coefficient of the covering layer.
Adaptation of the thermal expansion coefficient between an electrode and a dielectric plate in a wafer chuck is also known from U.S. Pat. No. 7,220,319 B2. The electrode is there arranged between a covering layer and a moderation layer, each of the thermal expansion coefficients of which lies between those of the electrode and of the dielectric plate.